1. Field of the Invention
The present invention relates to an image display apparatus for displaying an image, and in particular an image projecting apparatus for projecting an image formed on a display device onto a projection surface with an illumination light from a light source in accordance with input image data, such that the image can be observed by an observer.
2. Description of the Related Art
As an image display apparatus for displaying an image, an apparatus is provided which uses a display device such as a liquid crystal or a micro mirror to control the transmission amount or reflection amount of an illumination light from an illumination device, modulate the illumination light, and form and display a gray-scale image. A liquid crystal monitor, a projector and the like are provided as the above apparatus. To display a color image, as is often the case, illumination light components of primary colors are separately modulated, and are spatially combined or are combined while being emitted at different timings, thereby forming a color image. When a color image is displayed, it is necessary to adjust the combination ratio of the light components of primary colors with respect to balance, in order to ensure a high color reproducibility. Thus, generally, when input image data items regarding the primary colors are the same as each other, a so-called “white balance” is fixedly adjusted such that the combination of the colors looks white.
In general, illumination light components of primary colors are generated by fixedly separating light components of primary colors from light emitted from a white-light lamp by using a color separation optical element such as a dichroic mirror or a color filter. Thus, the illumination amount of the light components of primary colors cannot be flexibly controlled. Therefore, at an initial stage, the balance of the light components of primary colors is optically set to satisfy a predetermined ratio, thereby adjusting the white balance. Alternatively, the amount of modulation by the display device based on the input image data is corrected according to a predetermined conversion rule, thereby adjusting the white balance.
On the other hand, the upper limit of the brightness of illumination light or that of a displayed image obtained due to modulation by a display device can be more reliably set to the maximum, when the image is formed with illumination light components of primary colors the outputs of which are each set at the maximum. However, in general, there are no light sources which emit illumination light components of primary colors such that their maximum outputs are “white-balanced” by chance. Thus, in the above case, the white balance is lost as explained above, and inevitably the color reproducibility lowers. That is, in order to ensure that the brightness of the illumination light is the maximum, a high color reproducibility cannot be ensured, and in order to obtain a high color reproducibility, the light source cannot be made to emit the maximum amount of illumination light.
As a method for solving such a problem, a method disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-51353 is known. According to the method, only when the gradation levels indicated by image data items regarding primary colors which are included in the input image data are all the maximum or the minimum, an image is displayed by illumination light components of primary colors the outputs of which are the maximum. In the other cases, it is displayed in such a way as to maintain a predetermined white balance. Therefore, when the above gradation levels are all the maximum or minimum, the brightness of the displayed image is the maximum or minimum, but the color balance of the image is lost. Thus, generally, such a state is not recognized as a state in which a white balance is maintained. However, the brightness of the image can be increased without relatively worsening the color balance.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 2002-82652 discloses a so-called plane sequential type of image display apparatus, and an embodiment of the apparatus in which white illumination is performed each time light of each of primary colors is emitted. In the plane sequential of image display apparatus, illumination light components of primary colors are successively emitted onto a display device, and they are combined into an image to be displayed, while being viewed with observer's eyes. The method disclosed in the Publication is intended to improve the brightness of a produced image by emphasizing a white image component corresponding to a white image data item included in input image data. In a number of conventional plane sequential system of image display apparatuses, no image is displayed at the time of effecting switching between illumination light components of primary colors and between modulated images at a display device which correspond to the illumination light components, in order to prevent lowering of the quality of a displayed image, which would occur due to mixing of the color components at the time of effecting the above switching. However, the time for which illumination light is applied is shortened by the time for which no image is displayed, thus lowering the brightness of the displayed image. The technique of Jpn. Pat. Appln. KOKAI Publication No. 2002-82652 is intended to solve such a problem. However, in the technique of the Publication, the time period for which each of light components of primary colors is applied and that for which white illumination is performed are fixedly set at predetermined time periods.
The apparatus which is of such a plane sequential type as described above is not limited to an image display apparatus. To be more specific, there are provided plane sequential type of apparatuses which adjust and set the balance of the amounts of illumination light components of primary colors in accordance with various purposes. For example, in such a plane sequential type of electron endoscope as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-112962, the balance of illumination light components of primary colors is adjusted and set to correct the unbalance of the spectral sensitivity of an image pickup sensor.
The techniques disclosed in the above Publications are intended to increase the upper limit of the brightness of an image displayed by an image display apparatus, without excessively worsening the color balance of the image, and to obtain an image with a high reproducibility by adjusting the color balance of illumination light, thus adjusting the characteristics of an image pickup system.